1. Field of the Invention
The present invention relates to a vehicle navigation apparatus which is designed to search for a guidance route to a destination and to appropriately provide guidance information for guiding a vehicle along the guidance route.
2. Description of the Related Art
Conventional vehicle navigation apparatuses include a map-data storage device for storing therein map data, such as a digital versatile disk (DVD) or a hard disk, a display device such as a liquid crystal panel, and a vehicle position detector for detecting a present position of the vehicle and a present traveling direction thereof, such as a gyro, a global positioning system (GPS) receiver, and/or a vehicle speed sensor. In the navigation apparatus, map data including information around the present vehicle position is read from the map-data storage device. Based on the map data, a map image covering the present vehicle position and its surroundings is drawn and displayed on the display device with a vehicle position (location) mark superimposed on the map image. The map image is scrolled, or the vehicle position mark is moved with the map image fixed on a screen, in accordance with movement of the vehicle, thus allowing a user to understand the present traveling position of the vehicle at a glance.
Vehicle navigation apparatuses generally have the function of route guidance that enables the user to easily travel along an appropriate route to a desired destination without taking a wrong path. This route guidance function automatically searches for a route with the lowest cost connecting a starting point and the destination using the map data by a simulation calculation, such as the lateral search method or the Dijkstra method, and the resultant route is stored as a guidance route. During travel, the guidance route is drawn and displayed on the map by a thick line in a color different from that of other roads. Alternatively or additionally, when the vehicle is approaching an intersection on the guidance route at which it is to make a turn (i.e., at which it is to turn right or left), an arrow representing a traveling direction to be taken at the intersection is drawn on the map image, thereby guiding the user toward the destination.
The cost, as the term is used herein, may represent a value obtained by multiplying a distance of a road by a constant depending on the width of the road, the type of the road (general road, or highway), the direction of a turn, namely a right turn or a left turn, or the like. Alternatively or additionally, the cost may represent a time required for the vehicle to pass through the road on the basis of the distance of the road. That is, the cost may be a numeric value indicating the degree of appropriateness of the guidance route. Even if there are two roads with the same distance, the costs of the respective roads depend on, for example, whether the user makes use of a highway or not, or whether the user gives a high priority to the distance or the time, and thus may be different from each other.
Generally, the vehicle navigation apparatus displays an intersection guidance map on the display device, for example, when the vehicle is approaching an intersection on the guidance route at which it is to turn right, while providing voice guidance, for example, “Turn right at the next intersection”. On the intersection guidance map, roads located around the intersection are displayed in the form of a simple figure, and facilities located in the vicinity of the intersection are displayed as landmarks. Driving according to the voice guidance and/or the intersection guidance map enables the user to reach the destination with safety and comfort. Hereinafter, the intersection at which the vehicle is to make a turn, that is, the intersection of interest for guidance, is referred to as a guidance intersection, while an intersection other than the guidance intersection is referred to as a non-guidance intersection.
However, when the non-guidance intersection is located before the guidance intersection, the user may wrongly make a turn at the non-guidance intersection and not at the guidance intersection, thus causing the vehicle to depart from the guidance route.
U.S. Pat. No. 5,904,728 discloses a vehicle navigation system which is adapted to change the timing of providing voice guidance or to alter the contents of voice guidance, depending on whether or not a non-guidance intersection exists between a present vehicle position and a guidance intersection. Even in the vehicle navigation system disclosed in this patent, however, the user may miss the voice guidance, disadvantageously causing the vehicle to make a turn at the non-guidance intersection by mistake.